\name{formatci}
\alias{formatci}
\title{Format a confidence interval in parenthetic notation}
\description{Format a confidence interval in parenthetic notation}

\usage{formatci(ci, style=c("+/-", "parentheses"), model, digits=NULL)}

\arguments{
  \item{ci}{optional vector of length 2 or 3.}
  \item{style}{string indicating notation to be used.}
  \item{model}{optional regression model, e.g. returned by
    \code{\link{lm}} or \code{\link{nls}}.}
  \item{digits}{optional number of digits to use; if not supplied,
    \code{\link{getOption}("digits")} is used.}
}

\details{If a \code{model} is given, then \code{ci} is ignored, and a
  confidence interval is calculated using \code{\link{confint}} with
  \code{level} set to 0.6914619.  This \code{level} corresponds to a
  range of plus or minus one standard deviation, for the t distribution
  and a large number of degrees of freedom (since \code{qt(0.6914619,
    100000)} is 0.5).

  If \code{model} is missing, \code{ci} must be provided.  If it
  contains 3 elements, then first and third elements are taken as the
  range of the confidence interval (which by convention should use the
  \code{level} stated in the previous paragraph), and the second element
  is taken as the central value.  Alternatively, if \code{ci} has 2
  elements, they are taken to be bounds of the confidence interval and
  their mean is taken to be the central value.

  In the \code{+/-} notation, e.g. \eqn{a \pm b}{a +/- b} means that the
  true value lies between \eqn{a-b}{a-b} and \eqn{a+b}{a+b} with a high
  degree of certainty.  Mills et al. (1993, section 4.1 on page 83)
  suggest that \eqn{b}{b} should be set equal to 2 times the standard
  uncertainty or standard deviation.  JCGM (2008, section 7.2.2 on pages
  25 and 26), however, suggest that \eqn{b}{b} should be set to the
  standard uncertainty, while also recommending that the \eqn{\pm}{+/-}
  notation be avoided altogether.

  The \code{parentheses} notation is often called the compact notation.
  In it, the digits in parenthese indicate the uncertainty in the
  corresponding digits to their left, e.g. 12.34(3) means that the last
  digit (4) has an uncertainty of 3.  However, as with the
  \eqn{\pm}{+/-} notation, different authorities offer different advice
  on defining this uncertainty; Mills et al. (1993, section 4.1 on page
  83) provide an example in which the parenthetic notation has the same
  value as the \eqn{\pm}{+/-} notation, while JCM (2008, section 7.2.2
  on pages 25 and 26) suggest halving the number put in parentheses.

  The \code{foramtci} function is based on the JCM (2008) notation,
  i.e. \code{formatci(ci=c(8,12), style="+/-")} yields \code{"10+-2"},
  and \code{formatci(ci=c(8,12), style="parentheses")} yields
  \code{"10(2)"}.
  
  \strong{Note:} if the confidence range exceeds the value, the
  \code{parentheses} format reverts to \code{+/-} format.
}

\value{If \code{ci} is given, the result is a character string with the
  estimate and its uncertainty, in plus/minus or parenthetic notation.
  If \code{model} is given, the result is a 1-column matrix holding
  character strings, with row names corresponding to the parameters of
  the model.}

\examples{
x <- seq(0, 1, length.out=300)
y <- rnorm(n=300, mean=10, sd=1) * x
m <- lm(y~x)
print(formatci(model=m))
}

\author{Dan Kelley}

\references{JCGM, 2008.  \emph{Evaluation of measurement data - Guide
    to the expression of uncertainty in measurement (JCGM 100:2008)},
  published by the Joint Committee for Guides in Metrology.
  [\url{http://www.bipm.org/en/publications/guides/gum.html}]
  (See section 7.2.2 for a summary of notation, which shows equal values
    to the right of a \code{+-} sign and in parentheses.)
  
  I. Mills, T. Cvitas, K. Homann, N. Kallay, and K. Kuchitsu, 1993.
  \emph{Quantities, Units and Symbols in Physical Chemistry}, published
  Blackwell Science for the International Union of Pure and Applied
  Chemistry. [\url{http://old.iupac.org/publications/books/gbook/index.html}]
  (See section 4.1, page 83, for a summary of notation, which shows that
  a value to the right of a \code{+-} sign is to be halved if put in
  parentheses.)

}

\keyword{misc}
